Recently, optical reading such as bar-code reading, and the like, conducting scanning with a red LED (light emitting diode) or a laser light having a wavelength of from 620 nm to 700 nm has been practiced. Conventional bar codes typically have been formed by printing, heat-sensitive recording, and the like. However, more recently, pressure-sensitive recording sheets using microcapsules have been proposed which can carry out the above bar-code reading of an assembly constituted by a stack of two to six colored copy sheets.
In order to carry out optical reading with red light in a range of from 620 nm to 700 nm in wavelength, as described above, it is necessary that the colored dye used on the pressure-sensitive recording sheet has a strong absorption in the above-indicated red wavelength region. As compounds for forming dyes meeting this purpose, for example, the phthalide compounds described in JP-A-61-87758 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") can be used.
The present inventors have investigated pressure-sensitive recording sheets using the phthalide compounds as described in JP-A-61-87758 and found that the following problems exist. For instance, when clays such as acid clay, active clay, etc., phenol resins, organic acids, etc., as described in JP-A-61-87758 are used as a developer, the color density obtained was insufficient, the dyes formed had inferior light fastness, the dyes decomposed, especially in sunlight, and did not show the desired absorption.